1. Field of the Disclosure
Embodiments disclosed herein generally relate to blowout preventers used in the oil and gas industry. Specifically, selected embodiments relate to an improved seal carrier for use in ram-type blowout preventers, in which the seal carrier is configured to be displaced along an axis of the ram-type blowout preventer.
2. Background Art
Well control is an important aspect of oil and gas exploration. When drilling a well, for example, safety devices must be put in place to prevent injury to personnel and damage to equipment resulting from unexpected events associated with the drilling activities.
Drilling wells involves penetrating a variety of subsurface geologic structures, or “formations.” Occasionally, a wellbore will penetrate a formation having a formation pressure substantially higher than the pressure maintained in the wellbore When this occurs, the well is said to have “taken a kick.” The pressure increase associated with a kick is generally produced by an influx of formation fluids (which may be a liquid, a gas, or a combination thereof) into the wellbore. The relatively high-pressure kick tends to propagate upwards from a point of entry in the wellbore towards the surface (from a high-pressure region to a low-pressure region). If the kick is allowed to reach the surface, drilling fluid, well tools, and other drilling structures may be blown out of the wellbore. Such “blowouts” may result in catastrophic destruction of the drilling equipment (including, for example, the drilling rig) and substantial injury or death of rig personnel.
Because of the risk of blowouts, devices known as blowout preventers (“BOPs”) are installed above the wellhead at the surface or on the sea floor in deep water drilling arrangements to effectively seal a wellbore until active measures can be taken to control the kick. There are several types of blowout preventers, the most common of which are annular blowout preventers (including spherical blowout preventers) and ram-type blowout preventers. Blowout preventers may be activated so that kicks are adequately controlled and “circulated out” of the system. In deep water drilling, BOPs are conventionally used in an assembly called a “subsea BOP stack”, or simply a “subsea stack”, so-called because a number of BOP are “stacked-up” (that is, joined together) in an assembly, commonly with 4, 5, or 6 ram-type BOPs stacked-up below one or two annular BOPs. The large number of BOPs in a subsea stack affords redundancy which, for example, may allow the stack to remain on the seabed for an extended period.
Referring initially to FIG. 1, a schematic of a subsea BOP stack 10 is shown Subsea BOP stack 10 includes a lower double ram BOP assembly 11, a middle double ram BOP assembly 12, and an upper double ram BOP assembly 13. Furthermore, subsea BOP stack 10 includes spools 14 and an annular BOP 15. Each double ram assembly comprises two ram BOPs (or “cavities”) in a single body; consequently this stack comprises the equivalent of six conventional “single” ram BOPs and would be said to “have six ram cavities”.
Referring now to FIG. 2, an example of a typical conventional ram-type blowout preventer 100 is shown. Ram-type BOP 100 includes a BOP body 102 having a vertical bore 104 (i.e., the “wellbore”) and a horizontal bore 106. Vertical bore 104 is disposed about a vertical axis 105, and horizontal bore 106 is disposed about an axis 107 substantially perpendicular to axis 105. A joint of pipe 111 is shown disposed in vertical bore 104. Ram-type blowout preventer 100 further includes ram blocks 108 disposed within horizontal bore 106 on opposite sides, attached to piston actuated rods 112, and bonnets 110 which may be removably secured to BOP body 102 to enable removal of bonnets 110 for maintenance.
When ram-type blowout preventer 100 is actuated, ram blocks 108 displace along horizontal axis 107 toward vertical bore 104. Rams blocks 108 may either be pipe rams (shown) or variable bore rams, shear rams, blind rams, or any other known to those having ordinary skill in the art. Pipe and variable bore rams, when activated, move to engage and surround drillpipe and/or well tools to seal the wellbore. In contrast, shear rams engage and physically shear any wireline, drillpipe, and/or well tools in vertical bore 104, whereas blind rams close vertical bore 104 when no obstructions are present. More discussion of ram blowout preventers may be found in U.S. Pat. No. 6,554,247, issued to Berckenhoff, assigned to the assignee of the present invention, and incorporated herein by reference in its entirety.
As with any tool used in drilling oil and gas wells, blowout preventers must be sealed and secured to prevent potential hazard to the surrounding environment and personnel. For example, ram-type blowout preventers may include high-pressure seals between the bonnets and the body of the blowout preventer to prevent leakage of fluids. In many instances, the high-pressure seals are elastomeric seals and should be checked regularly to ensure that the elastomeric components have not been cut, permanently deformed, or deteriorated by, for example, a chemical reaction with the drilling fluid in the wellbore.
Referring still to FIG. 2, ram-type blowout preventer 100 includes top seals 116 disposed within grooves 118 of ram blocks 108, which are sealingly connected to front seals (or “ram packers”) 109. In the case of pipe rams, when ram blocks 108 are closed as shown, the combination of the top seals 116 (which seal between the top of ram blocks 108 and the top of horizontal bore 106), and the front seals 109 (which seal completely around pipe in the vertical bore 104) completes the sealing of the annulus between the pipe 111 and the vertical bore 104. In the case of shear rams and blind rams, when ram blocks 108 are closed, the front seals 109 seal against one another rather than an object in the wellbore, and the combination of the top seals 116 and the front seals 109 completes the sealing-off of the open wellbore,
Conventionally, ram blocks 108 have a pressure equalization path in the form of a groove 101 (sometimes called a “mud slot”) machined into the bottom surface of the ram block to communicate fluid pressure between the vertical bore 104 below the front seals 109 and the respective volumes of the horizontal bore 106 behind the ram blocks. Thus each ram block 108 may be displaced back and forth in the horizontal bore 106 without having to work against fluid pressure differentials between the volume behind the ram blocks 108 and the vertical bore 104 below the front seals 109. Those skilled in the art will of course recognize that fluid pressure communication for pressure equalization between the vertical bore 109 and the volumes behind the ram blocks 108 may be accomplished by other means besides a machined groove in the bottom of the ram blocks 108, such as drilled passageways in the ram blocks, a milled slot in the bottom of the horizontal bore 106, or even a conduit external to the housing 102, or the like.
Referring now to FIG. 3, an enlarged cross-sectional view of a top seal wear plate 120 of a ram BOP is shown. Because the top surface of horizontal bore 106 may wear with repeated use of the ram BOP, modem ram BOPs may be fitted with replaceable top seal wear plates to avoid expensive repair of the horizontal bore 106. Top seal wear plate 120 is immovably secured to housing 102 with, for example, bolts 122 and collet-type inserts 122A, and includes a sleeve portion 120A and a flange portion 120B extending radially outward with respect to wellbore axis 105. Additionally, as shown, top seal wear plate 120 is adjacent to ram block 108 and seals against housing 102 to prevent, in conjunction with top seal 116, leakage between housing 102 and ram blocks 108. Typically, an o-ring 124 is disposed in a groove 126 of flange portion 120B of top seal plate 120 to sealingly engage (as a face seal) against housing 102 and prevent leakage of high-pressure fluids between housing 102 and seal carrier 120.
Since the primary function of ram-type BOPs is to prevent the escape of fluids from the wellbore, many ram-type BOPs only seal in a single direction. Thus, a ram-type BOP may only seal to isolate pressurized fluids from the wellbore to the environment and will not typically include a bidirectional seal; capable of sealing against a differential pressure from above the BOP.
For example, when ram blocks 108 are engaged with one another and sealing against high-pressure fluids from above, the high-pressure fluids may act upon the top surface of ram blocks 108 and urge them downward. Such urging may cause ram blocks 108 to move downward and out of sealing engagement with the top of the horizontal bore 106 (or alternately, in a ram BOP so equipped, with seal carrier 120).
Formerly, deploying a ram-type BOP having a single direction seal was not considered to be a shortcoming, as there was no reason for a BOP to seal against pressure from above. However, it is now common in deepwater drilling installations for regulatory agencies (e.g., the Minerals Management Service (“MMS”), of the United States Department of the Interior, which regulates offshore drilling for oil and gas in U.S. territorial waters) to require periodic testing of the integrity of individual ram BOPs against wellbore pressures while the subsea stack is located on the seabed.
Previously, such in situ BOP testing may have been accomplished through one of two test methods. In a first test method, a test tool is lowered through the subsea BOP stack on a string of pipe, and anchored below the lowest BOP in the stack. The test tool is actuated to seal off the wellbore at that point (as, for example, by inflating an inflatable packer), and a BOP to be tested is closed. Then, fluid pressure is communicated into the annular space around the pipe above the test tool and below the BOP being tested. After testing, the pipe string and test tool are withdrawn from the wellbore, and normal drilling operations can be resumed.
However, such a method may be extremely costly in terms of rig time. In an alternative test method, the subsea BOP stack may include an additional ram BOP installed in an inverted operating position at the bottom of the subsea BOP stack. Thus, the inverted BOP may seal against test pressure introduced thereabove. However, in placing the additional BOP in an inverted position at the bottom of a subsea BOP stack, the additional ram “cavity” may will not seal against wellbore pressure and thus may not be used as a regular BOP during operations. Furthermore, the additional BOP may also increase the height, weight and cost of a subsea BOP stack.
Consequently, it may be advantageous to have a ram BOP having the ability to seal in both directions on the seabed so that the BOP stack may be tested without running a dedicated test tool into the well. Furthermore, such a dual direction ram BOP will allow the BOP stack to be tested without requiring a dedicated inverted cavity for testing purposes. Further, because of the large number of subsea BOP stacks in existence, it may also be advantageous to have an inexpensive apparatus and method to modify existing ram-type BOPs so they could seal in both directions.
One device currently capable of effecting a bi-directional seal of a bore or conduit, for example in a gate or ball valve, involves separate seals (either metal-to-metal seals or deformable seals) on either side of a movable pressure barrier, whereby each seal acts independently of the other to seal-off pressure from one direction or the other.
However, ram BOPs attempting this “double-seal” approach may disadvantageously trap pressurized fluid behind the ram block, thereby effectively hydraulically locking the ram block. Additionally, the “bottom” sealing mechanism may add complexity and manufacturing expense. Furthermore, because the heavy weight of the ram blocks and the abrasive nature of the wellbore fluid, such a on a ram BOP may have limited working life.
A ram BOP having bi-directional sealing rains is disclosed in U.S. Pat. No. 4,655,431, issued to Helfer, et al, and incorporated by reference herein in its entirety. The ram BOP of Heifer comprises circumferential seals around the ram blocks wherein passages within the ram blocks between the face and rear of the ram blocks, both above and below the front seal, and valve means within the ram blocks allow flow through the passages only from the front to the rear of the ram block. Such a design is alleged to hold pressure equally from either direction.
Additionally, a ram BOP having bidirectional sealing rams is disclosed by U.S. Pat. No. 6,124,619, issued to Van Winkle, et al, and incorporated herein by reference in its entirety. In lieu of conventional seals, the ram BOP of Van Winkle includes ram block seals which go all the way around the ram block to seal the space behind the rams. In addition, a mechanism is provided to selectively connect the volume behind the rams with the more highly pressurized wellbore volume adjacent to the rams (either above or below). The connection made is free-flowing in both directions thereby allowing for evacuation and fluctuations with changes in wellbore pressure.
Furthermore, a BOP having bidirectional sealing rams is taught is U.S. Pat. No. 6,719,262, issued to Whitby, et al and incorporated herein by reference in its entirety. This BOP of the Whitby patent includes top seals and bottom seal and, in order to mitigate the issues of fluid trapped behind the ram blocks, includes two fluid communication systems. The first communication system is a selectively operable system to equalize the pressure behind the back of each ram with the fluid pressure below the ram packers. The second communication system includes a selectively operable fluid communications system for equalizing fluid pressure between the back of each ram with the fluid pressure above the ram packers. As such, each selectively operable fluid control system includes a control unit connected to it for such “selective” operation.
All prior-art solutions rely on completely sealing-off the ram block within the horizontal bore and equalizing the pressure differential between the wellbore (above or below the ram blocks) and the volumes behind the ram blocks. These systems are relatively complicated and expensive, the pressure balancing passageways may be prone to plugging (e.g., by drilled cuttings in the drilling mud), and failure of certain pressure-equalizing valve components may provide an open conduit from the wellbore below the ram blocks to the wellbore above the ram blocks. More critically, if the pressure-equalization mechanisms fail (whether, for example, by plugged passageways or the failure of a valving component) while operating in a subsea stack, the cessation of drilling operations, killing the well, and pulling the entire subsea stack to the surface for repairs would likely be required. Therefore, it would be desirable to have a bidirectional sealing ram BOP which does not require pressure equalization passages or valving. Additionally, it would also be desirable to have a ram BOP capable of sealing against bi-directional pressure using the existing ram block seals of “legacy” ram BOPs.